Shared aperture antenna array

ABSTRACT

A shared aperture antenna array including an array of antennas is disclosed. Elements of neighboring antennas are shared to create additional antennas. The shared elements include radiating patches and apertures. Each antenna shares an aperture with neighboring antennas. The array of antennas may be linear or two-dimensional. A phase shifting network with single-pole-single-throw reflective switches may be coupled to the antennas.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under ContractDE-AC05-76RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

This invention relates to antenna arrays. More specifically, thisinvention relates to a shared aperture antenna array wherein elements orportions of neighboring antennas are shared to create additionalantennas.

BACKGROUND OF THE INVENTION

Current technologies for linear and two-dimensional radar imagingantenna arrays use sequential sampling, synthetic apertures, and sparsearray techniques to achieve sampling/pixel density. All of thesetechniques focus on switching networks to use antennas in the array astransmitters/receivers. For example, linear holographic radar imagingarrays typically use two separate linear arrays of cavity-backed spiralantennas to transmit and receive circularly polarized signals.

Microwave and millimeter-wave technologies or synthetic aperture imagingtechniques have been developed for a wide variety of applications. Theseapplications include radar (GPR), through-wall and inner wall imaging,body measurements, security screening, and non-destructive evaluation.The imaging techniques developed are fully three-dimensional andtypically operate by scanning a wide bandwidth radar transceiver over aplanar or cylindrical aperture, and using mathematical techniques tofocus the data into a three-dimensional image. Examples of thesetechniques are described in U.S. Pat. Nos. 5,557,283 and 5,859,609 bySheen et al. It is advantageous to use mathematical focusing for theseapplications because it allows for the use of large apertures andextreme near-field operation, where it would be inconvenient orimpossible to use physical focusing elements such as lenses orreflectors. Additionally, scanning the transmitter along with thereceiver doubles the resolution relative to fixed transmitters andprovides superior illumination quality by using a large diversity oftransmitters.

Many near-field radar imaging applications require real-time ornear-real-time data collection and imaging. Sequentially-switched lineararray technology that allows one dimension of a planar or cylindricalaperture to be effectively scanned electronically at high speed has beendeveloped. This is accomplished by sequencing through each antennaelement or transmit/receive antenna pair using microwave ormillimeter-wave switching networks connected to the radar transceiver.

Phased array antenna systems are well known in the antenna art. Suchantennas are generally comprised of radiating elements that areindividually controllable with regard to relative phase and amplitude.The antenna pattern of the array is selectively determined by thegeometry of the individual elements and the selected phase/amplituderelationships among the elements. Typical radiating elements for suchantenna systems may include dipoles, patches, waveguides, or slots.

One example of a planar antenna element is known in the art as theFoursquare antenna, as described in U.S. Pat. No. 5,926,137 to Nealy. Itcomprises four square radiating elements on the top side of a dielectricsubstrate which is separated from a ground plane by a foam separator. Atleast two coaxial feeds connect to interior corners of opposing pairs ofradiating elements. This Foursquare antenna provides widebandperformance and several practical advantages for commercial and militaryapplications. Various polarizations can be achieved with the Foursquareantenna—for example, dual linear, circular and elliptical polarizationsof any orientation—and its features are a low-profile geometry andcompact radiating element size.

The continuing challenge with the development of antenna arrays used inholographic imaging applications is to reduce the size and weight andachieve adequate physical sampling/pixel spacing in the array. Therequired sampling is typically a minimum of one-half wavelength. Thephysical sampling of the array is determined by the spacing of transmitand receive antennas used in the array system. Since the physical sizeof the antenna is constrained by the bandwidth of operation, thephysical size of the antennas cannot be greatly reduced to achieve anynoticeable increase in sampling density.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, an antennaarray is disclosed. The antennas array includes an array of antennaswherein elements of neighboring antennas are shared to create additionalantennas.

The shared elements include radiating patches and apertures. Aperturesof neighboring antennas overlap.

In one embodiment, feed points of the antennas are phased so thatopposing dipole patches are 180 degrees out of phase.

In one embodiment, each antenna is a foursquare antenna.

The antennas are printed on a dielectric substrate and located above aground plane.

The antenna array includes a phase shifting network withsingle-pole-single-throw reflective switches coupled to the antennas.

The array of antennas is a linear array of antennas or a two-dimensionalarray of antennas.

In another embodiment of the present invention, a shared apertureantenna array is disclosed. The shared aperture array includes an arrayof antennas. Each antenna shares an aperture with neighboring antennas.

In another embodiment of the present invention, a method of increasingsampling density using a shared aperture antenna array is disclosed. Themethod includes providing an array of antennas; and creating additionalantennas by sharing elements of neighboring antennas within the array.

In one embodiment, the method includes electrically coupling a phaseshifting network with single-pole-single-throw reflective switches tothe antennas.

In another embodiment of the present invention, a shared apertureantenna array is disclosed. The shared aperture array includes an arrayhaving a plurality of antennas wherein each antenna shares at least oneradiating patch and an aperture with a neighboring antenna. At least onefeed point is phased so that opposing dipole patches are 180 degrees outof phase. The antennas are printed on a dielectric substrate and locatedabove a ground plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a shared aperture antenna linear array, inaccordance with one embodiment of the present invention.

FIG. 2 is a schematic of a phase shifting network coupled to antennas,in accordance with one embodiment of the present invention.

FIG. 3 is a graph showing antenna gain using the shared aperture antennaarray of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes an array of antennas that sharescomponents or elements of neighboring antennas to create additionalantennas. The shared aperture array creates additional antenna elementswithin an array of broadband planar dipole antennas, allowing forincreased array sampling and pixel density than current antenna arraydesigns. The shared aperture array can achieve pixel sampling distancesmuch smaller than one-half wavelength with a single array of antennas.The sharing of elements of neighboring antennas to create additionalantennas reduces the physical size of the arrays by a factor of two,allows for fully polarimetric data sets with a single array, increasesthe physical sampling—reducing pixel sampling in the array—toone-twelfth of a wavelength, and enables imaging systems with reducedweight.

In one embodiment, the present invention discloses an array of broadbandplanar dipoles and maintains desirable circular polarization propertiesof transmitted and received signals using an appropriate feed network.

FIG. 1 is a schematic of a shared aperture antenna array 100, inaccordance with one embodiment of the present invention. The antennaarray 100 includes a first antenna 110, a second antenna 120, and athird antenna 130. In this embodiment, each antenna 110, 120, and 130 isa foursquare antenna that includes four radiating patches or metalsheets 140. Each radiating patch 140 includes at least one feed point150. It should be noted that the antenna array 100 is not limited to anyparticular number of antennas, radiating patches or feed points. Notealso that the antenna array can be a single (linear) or atwo-dimensional array of antennas.

In the present invention, certain elements or components of the antennaarray are shared. The shared elements include the radiating patches 140and apertures. For example, in FIG. 1, the radiating patches 140 of thefirst antenna 110 and the third antenna 130 are shared to create thesecond antenna 120. As such, the same radiating patch is used for twoantennas, and two radiating patches belong to two antennas. This isachieved without changing the physical size of the array 100. The numberof antennas is increased as the radiating patches of neighboringantennas are shared. This results in having nearly twice as manyantennas in the same space compared to current or prior antenna arraydesigns.

Apertures of neighboring antennas also overlap. Still referring to FIG.1, the first antenna 110 has an aperture consisting of its fourradiating patches. Likewise, the second and third antennas 120 and 130each have apertures above them. However since the first antenna 110 andthe second antenna 120 overlap, they have a shared aperture. Similarly,since the second and third antennas 120 and 130 overlap, they also sharean aperture. Each antenna shares an aperture with neighboring antennas.

FIG. 2 is a schematic of a phase shifting network 200 coupled toantennas using single-pole-single-throw reflective switches, inaccordance with one embodiment of the present invention. The network 200includes two separate 180 degree hybrids or baluns plus a separate 90degree hybrid for each antenna. The feed points of each antenna arecoupled to the 180 degree hybrid couplers via single-pole-single-throwreflective switches. A transfer switch is coupled between the differencesignal outputs of the 180 degree hybrid couplers and the 90 degreehybrid coupler, which is used to produce circular polarization. A secondswitch is coupled between the 90 degree output port and atransceiver—the transmit switch and the receive switch—which is used forgenerating signals to be transmitted and processing received signals. Inone embodiment, the array transmits with right-hand circularpolarization (RHCP) and receives with left-hand circular polarization(LHCP).

The antenna array of the present invention was simulated and verifiedusing a finite element model of a planar broadband foursquarepatch/dipole array with a center frequency of approximately 10 GHz. FIG.3 is a graph showing the antenna gain using this array configuration,which is similar in design to the antenna array of FIG. 1.

Results of the simulation demonstrated the following: an increasedsampling/pixel density—e.g., one-twelfth wavelength sampling, which is3-6× better than existing array designs—reduced array surface by 50%,the ability to utilize co-polarized and cross-polarized signals with asingle array, and antenna gain by approximately 5 dB relative tocavity-backed spiral array designs.

Antenna geometries and feed network components are frequency scalable,so this design may be used over the RF, microwave, and mm-wavespectrums. The performance of the linear antenna array can beextrapolated using an infinite array model of the shared apertureantenna used in a 2-D antenna array configuration.

The shared aperture antenna array of the present invention was able tomaintain the desired antenna gain, voltage standing wave ratio (VSWR),and circular polarized properties of the baseline antenna by employing areflective switch at the input to the shared ports of the foursquareantenna patches to appear as a short-circuit when the element was notbeing used.

The shared aperture antenna array demonstrated the ability to obtainboth right-handed and left-handed senses of circular polarization usingonly a single array of antennas. Sampling configurations within thearray allow for improved physical sampling within the antenna array assmall as one-twelfth of a wavelength.

The present invention is applicable to any frequency band via frequencyscaling of the physical antenna dimensions.

Applications involving linear and 2-D holographic radar arrays include,but are not limited, to the following: security screening, handheldholographic imaging systems, ground penetrating radar (GPR), andclothing/apparel measurements.

The shared aperture array allows for increased physical samplingthroughout the antenna array, which results in high-fidelity andmultiple polarization state based images. High-fidelity images inconjunction with multiple polarization state data sets can increase theprobability of detection of threat objects and improve the accuracy forapparel measurements.

The present invention has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the invention. As such,references herein to specific embodiments and details thereof are notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications can be made inthe embodiments chosen for illustration without departing from thespirit and scope of the invention.

We claim:
 1. An antenna array comprising: an array of antennas, whereinelements of neighboring antennas are shared to create additionalantennas.
 2. The antenna array of claim 1 wherein the shared elementsinclude radiating patches and apertures.
 3. The antenna array of claim 2wherein feed points of the antennas are phased so that opposing dipolepatches are 180 degrees out of phase.
 4. The antenna array of claim 2wherein apertures of the neighboring antennas overlap.
 5. The antennaarray of claim 1 wherein each antenna is a foursquare antenna.
 6. Theantenna array of claim 1 wherein the antennas are printed on adielectric substrate and located above a ground plane.
 7. The antennaarray of claim 1 further comprising a phase shifting network withsingle-pole-single-throw reflective switches coupled to the antennas. 8.The antenna array of claim 1 wherein the array of antennas is a lineararray of antennas or a two-dimensional array of antennas.
 9. A sharedaperture antenna array comprising: an array of antennas, wherein eachantenna shares an aperture with neighboring antennas.
 10. The sharedaperture antenna array of claim 9 wherein the antennas are printed on adielectric substrate and located above a ground plane.
 11. The sharedaperture antenna array of claim 9 wherein apertures of neighboringantennas overlap.
 12. The shared aperture antenna array of claim 9wherein at least one radiating patch of each antenna is shared with theneighboring antennas.
 13. The shared aperture antenna array of claim 12wherein at least one feed point of each antenna is phased so thatopposing dipole patches are 180 degrees out of phase.
 14. The sharedaperture antenna array of claim 9 wherein each antenna in the array is afoursquare antenna.
 15. The shared aperture antenna array of claim 9further comprising a phase shifting network withsingle-pole-single-throw reflective switches coupled to the antennas.16. The shared aperture antenna array of claim 9 wherein the array ofantennas is a linear array of antennas or a two-dimensional array ofantennas.
 17. A method of increasing sampling density using a sharedaperture antenna array comprising: a. providing an array of antennas;and b. creating additional antennas by sharing elements of neighboringantennas within the array.
 18. The method of claim 17 wherein theantennas are printed on a dielectric substrate and located above aground plane.
 19. The method of claim 17 wherein the shared elementsinclude apertures and radiating patches.
 20. The method of claim 17wherein feed points of the antennas are phased so that opposing dipolepatches are 180 degrees out of phase.
 21. The method of claim 17 whereinapertures of the neighboring antennas overlap.
 22. The method of claim17 wherein each antenna is a foursquare antenna.
 23. The method of claim17 further comprising electrically coupling a phase shifting networkwith single-pole-single-throw reflective switches to the antennas.
 24. Ashared aperture antenna array comprising: an array having a plurality ofantennas, wherein each antenna shares at least one radiating patch andan aperture with a neighboring antenna, and wherein at least one feedpoint is phased so that opposing dipole patches are 180 degrees out ofphase, and wherein the antennas are printed on a dielectric substrateand located above a ground plane.